fishdescriptor: cast __errno_location correctly

fishdescriptor: cast __errno_location correctly

The return value from __errno_location() is an int*.  My syntax had
erroneously specified that it returned an int.  Nothing spotted this
bug because the expression is evaluated by gdb whose C interpreter is
very strange.  In particular, gdb lets you dereference an int, even on
a platform where ints are 32-bit and pointers are 64-bit.

If you are on a 32-bit platform, this does not matter.  Likewise if
you are on a 64-bit platform and the address of errno happens, by
luck, to be within the 32-bit addressable part of the space.

If you are mildly lucky the result of this is an error like this:
  gdb.MemoryError: Cannot access memory at address 0x2f24ef10
buried in stack traces from fishdescriptor.

If you are unlucky, fishdescriptor will successfully access some wrong
location.  This means it does not actually save and restore errno,
since it saves and restores somewhere else instead.  So fishdescriptor
will corrupt the errno value of the thread that it happens to
be (ab)using, overwriting it with the errno from fishdescriptor's own
calls, possibly causing the target program to become confused about
the error(s) from system call(s) it is making.

If you are very unlucky, fishdescriptor will successfully access some
wrong location which is actively in use by something outside the
target process (eg, direct IO, shared memory0.  fishdescriptor will
save the value (a single int) and then restore it a bit later.  This
might in principle cause any kind of arbitrary lossage.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>